The present invention relates to stereomicroscopes, and more particularly to stereomicroscopes used to detect differences between substantially identical photographs, either aerial photographs or photographic masks used by the semiconductor industry.
Chaban U.S. Pat. No. 3,879,107, issued Apr. 22, 1975, discloses a stereomicroscope of the type used to compare aerial photographs. The disclosed stereomicroscope has a pair of rhomboid arms used to position the respective objectives over the pertinent portion of a respective aerial photograph. Each rhomboid arm contains an independent zoom system to produce an intermediate image which is then relayed to an image rotator. A beam splitter selectively placed on the optical axes permits either mono or stereo presentation of the images.
Stereomicroscopes of the type disclosed in the Chaban patent are conventionally used to compare two aerial photographs or a mask with a master, in connection with the preparation of semi-conductor wafers, by placing one of the photographs or wafer or mask under one rhomboid arm and the other photograph or wafer or mask under the other rhomboid arm. The rhomboid arms are then moved until substantially the same object is located in each photograph. The fields of view are then angularly adjusted by the operator, using the image rotators, to rotate either or both images until they both have an identical orientation. The operator then adjusts either or both zoom systems until the sizes of the two images are identical. It may then be necessary to slightly reposition the rhomboid arms or one of the photographs or wafers or masks until the two fields are identical, which may also require a further adjustment of the image rotators. Although not disclosed in the Chaban patent, it is conventional in such stereomicroscopes to permit superimposition of the images. That is, the image from one rhomboid arm is presented to both eyepieces and the image from the other rhomboid arm is presented to both eyepieces. Many operators prefer to use this superimposed mode to conveniently obtain proper orientation, image size an identical fields.
Prior to the present invention, differences between substantially identical photographs were identified by an operator using mental recall. After the orientation, size and identical fields had been selected using a procedure of the type described above, an operator would then switch to the mono mode, that is, viewing one photograph through one eyepiece and then switching to view the other photograph through the other eyepiece, while relying on mental recall to detect an observed difference. Some operators preferred to use the biocular mode, if available, i.e. view one photograph through both eyepieces and then the other photograph through both eyepieces. This still depended upon the operator to recall specifics of one image while viewing another. Yet another possibility involved viewing one stereo pair of photographs on one stereo viewing instrument and viewing a second stereo pair of photographs of the same scene on another stereo viewing instrument and again using mental recall attempt to detect differences between the photograph stereo pairs. This method is especially cumbersome and requires two viewing instruments. This prior art procedure had the obvious disadvantage that it was highly dependent upon operator skill and the operator's ability to recall precise details of the prior image while viewing the subsequent image.